In motor vehicles, the use of a filtering pulley for driving the alternator via the accessory drive belt is known.
This pulley comprises a hub suitable for being connected to the rotor of the alternator, an annular crown provided with a profile suitable for cooperating with the belt and a filtering unit interposed between the hub and the crown of the pulley to avoid subjecting the belt to dynamic overloads in the presence of torsional vibrations of the driveshaft or rapid variations in engine speed.
The filtering unit generally comprises a spring, for example a coil spring (helical or spiral), or a plurality of springs or other elastic elements arranged circumferentially between the hub and the crown.
In addition, since the alternator has relatively high inertia and tends to drive the pulley during instantaneous drops in driveshaft speed, it is known to incorporate a freewheel in the filtering unit that is able to transmit motion unidirectionally from the crown to the hub (normal operating condition) and disengage the hub from the crown in the event of torque reversal. The freewheel can be implemented, for example, by means of a band spring carried by either the hub or crown and able to expand or contract according to the torque direction to cooperate with, or disengage from, a corresponding annular friction surface integral with the other of the hub and crown.
The introduction of start-stop systems in which the alternator is replaced by a reversible electric machine having the function of starter motor or alternator according to the operating conditions makes it impossible to use a filtering pulley with a freewheel, because this would disengage the electric machine from the belt drive when the machine provides power.